Avalanche Forecast
Regions: Olympics.
Avoid backcountry travel on slopes with previous firm wind slab where you could trigger the slab from an area where the wind slab is shallower. Watch for new wind slab that is expected to form during the day on Sunday.
Detailed Forecast
A moderate warm front will move south to north over the Northwest on Sunday. S-SE winds should increase on Sunday over the Olympics. A few inches of new snow is expected by the end of the daylight hours with a warming trend.
Watch for new wind slab mainly on northwest to southeast slopes in the near and above treeline on Sunday. Previous wind slab will linger on other slope aspects in all the terrain bands and will have been slow to heal due to the cold temperatures.Watch for firmer wind transported snow.
Continue to identify the 12/17 buried PWL in snowpits and avoid areas where the overlying snowpack is shallower where affecting this layer would be more likely.Â
Snowpack Discussion
Weather and Snowpack
A cold dry Arctic air mass was over the Northwest last week. The main weather event last week was a day or so of east winds on Wednesday that went on a powder wrecking rampage, especially in the Cascades but also in the Olympics, scouring windward slopes, and re-distributing it to lee slopes or other areas.
It sounds like the winds may have eliminated much of the faceted surface snow and surface hoar that may have formed out there in the near and above treeline during the cold weather. But these weak surface crystals may still be found in sheltered areas below treeline and will need to be avoided where buried by snowfall especially above terrain traps.
 Recent Observations
NWAC pro-observer, Matt Schonwald was in the field with NPS rangers in the Hurricane Ridge area Friday, 1/6. Matt traveled to the Steeple Rock area along the Obstruction Peak route. The persistent weak layer from 12/17 was very evident in the two distinct areas where snow tests were performed. The layer ranged in depth from 1105 cm on SSW aspect to 135 cm on NW aspect. While reactive in deep snowpack tests and PST's, stiff overlying layers and being buried greater than 1 meter are indicating triggering this layer is unlikely. Of greater concern were the areas of recent wind slabs on a variety of aspects. There was no evidence of very recent avalanches, in that terrain, but older slides were seen, likely releasing during the strong wind events earlier in the week.
Avalanche Problems
Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1